Analog telephone communication or POTS (plain old telephone service) typically requires a bandwidth of approximately 4 KHz. The existing twisted pair copper wire infrastructure now in place and initially designed for analog voice communication is now being viewed by telephone companies as a means of delivering high-speed digital information, usually over the "last mile" (i.e., the last segment of copper wire between a central office and a user location). To accommodate the high speed digital data over the same copper wire as the POTS service, a pair of modems are used, one at a central location (the central office or optical network unit (ONU)) and another at the user or residential premise, called a digital-subscriber-line (DSL) modem. In the case of DSL, the digital information is communicated over the same copper wire as the POTS service, but at higher frequencies, with the lower portion of the frequency spectrum being reserved for POTS service in the event of a failure of the high speed communication system. Passive filters are used to separate the POTS service and the high speed data which occupies the frequency spectrum from approximately 30 kHz up to 10 MHz. This feature is commonly referred to as "lifeline POTS service" and is generally advantageous to maintain as part of a communication system due to the time-tested reliability of POTS service.
While there are several versions of DSL, one type is an Asymmetric DSL (ADSL) communication system which is intended for consumer applications such as Video on Demand and Internet access. ADSL provides bidirectional data communication between the central office or "CO" and several end user or remote locations. In such an asymmetric system, the "downstream" data direction is defined as the transmission of data from the central office or ONU to the remote locations, while the "upstream" data direction is defined as the transmission of data from a remote location to the central office. Because the bandwidth capacity of the copper wire is somewhat limited, the ADSL asymmetric data communication system allocates more of the available frequency spectrum to the downstream direction. This allocation is based primarily on the fact that more data generally flows in the downstream direction than in the upstream direction. For example, one application of this type of system may be "video on demand" where an end user at a remote location requests the transmission of a particular video program. In this situation, the upstream data consists primarily of control and selection information, whereas the downstream data is significantly larger in that it consists of large amounts of data intensive video information. Thus, the upstream and downstream channels need not be equal for most, if not all, applications.
The transmit signal frequency spectrum allocation for a conventional CAP (Carrierless AM/PM) based Rate Adaptive DSL (RADSL) system utilizing the existing copper wire infrastructure, which has been proposed by AT&T Paradyne to the T1.E1.4 Committee, is shown in FIG. 1. As shown in FIG. 1, POTS communication occupies the lowest portion of the spectrum, typically 4 KHz. The next portion of the spectrum is allocated to the upstream channel. The spectral start frequency for the upstream channel is 35 kHz and the baud rate is fixed at 136 kbaud for modulations of 8, 16, 32, 64, 128 and 256 CAP. The next portion of the spectrum is allocated to the downstream channel. The spectral start frequency for the downstream channel is 240 kHz and the baud rate is selectable from 340, 680, 816, 952, and 1088 kbaud for modulations of 8, 16, 32, 64, 128 and 256 CAP. The upstream and downstream channels both use nominal square-root raised cosine shaping. The selection of baud rate and modulation format is determined during an initialization sequence where an exhaustive search is performed to locate the best configuration for the given loop and noise environment.
However, there are several drawbacks to the above mentioned implementation. First of all, it is only effective if the interference and noise environment remain fairly static, since the line conditioning is only performed during the initialization sequence. Second, a single large narrowband interference which is typical for RFI may result in a significant reduction in available modulation states required to maintain the necessary BER margin.
The present invention overcomes these drawbacks by employing a robust technique for line probing and characterization which allows a near-real time allocation of the upstream and downstream spectra, while at the same time optimizing the baud rate and modulation format.